[tgflynn]

You didn't address this part of the parent comment:

> I imagine it'd pull in electrons from all around itself, but I don't know how the numbers come out.

I never thought of it before but it seems like that should work. "Space" is actually a neutral plasma, right, so it should be full of free electrons. Those should neutralize the ship before any significant charge builds up. It seems like you should be able to use space itself (or more accurately the interplanetary medium) as a massive ground plane to complete the circuit for the charged exhaust beam.

[e_y_]

Space is pretty empty. From what I can find, the interplanetary medium is around 5 particles/cm^3 compared to the exhaust from the ion thruster which seems to be around 10^6 particles/cm^3 and disperses to 10^4 particles/cm^3 further away.

Source: https://trs.jpl.nasa.gov/handle/2014/15643

"As the charge-exchange plasma density near spacecraft is at least about three orders of magnitude larger than the solar wind plasma density, the plasma environment of DS1 spacecraft is completely dominated by the charge-exchange plasma in the plume."